This application claims the benefit of Taiwan application Serial No. 092104599, filed Jan. 16, 2003.
1. Field of the Invention
The invention relates in general to a light emitting diode (LED), and in particular, to an LED having a BP (boron phosphide) buffer layer of single crystal structure on a ZnTe or ZnSe substrate to facilitate the epitaxy of a blue light LED of cubic crystal structure on the BP buffer layer.
2. Description of the Related Art
The light emitting diode (LED), with the characteristics of small volume, light weight, high efficiency, and long life, has seen great advances in different monochromatic color output, such as red, blue, and green. Single color LED""s can be used as a backlight in a special display, for instance, mobile phones and light crystal displays (LCDs). Nowadays, many researches are being carried out on white LEDs in order to develop illumination apparatus with higher efficiency than traditional incandescent and fluorescenct devices.
FIG. 1 shows the cross section of an LED of lateral-electrode type, wherein the P-type electrode and N-type electrode are positioned on the same side of the substrate. A first cladding layer, such as N-type GaN layer 11, is on a substrate of sapphire layer 10. The LED further comprises a buffer layer, not shown in the figure, between the substrate 10 and the first cladding layer 11. An active layer, such as GalnN layer 12, is on the first cladding layer 11. A second cladding layer of P-type GaN layer 13 is on the active layer 12. An N-type electrode 14 and a P-type electrode 15 are respectively on the first cladding layer 11 and the second cladding layer 13.
U.S. Pat. No. 5,998,925 discloses a white color LED, wherein the stack structure described in the previous paragraph is packaged with a phosphor, like a YAG phosphor 16. The blue light emitted from the active layer is partially absorbed by the YAG phosphor 16. The unabsorbed blue light is mixed with yellow light emitted from YAG phosphor 16, and thus the white light is produced with this mixing.
However, the quality of the white light produced is not satisfactory if the intensity ratio between the blue light and the yellow light is not equal. To form the phosphor layer covering the stack structure greatly increases the cost of the packaging. Moreover, the cost also increases because the substrate of sapphire layer 10 is insulation and the N-type electrode has to be formed on the first cladding layer by an additional etching step.
To resolve the issues described above, an LED of vertical-electrode type has been developed and is shown in FIG. 2, wherein the P-type electrode 28 and N-type electrode 21 are positioned on the different side of substrate. Moreover, the substrate, such as an N-type SeZn substrate 22 of FIG. 2, is capable of transferring the wavelength of the light source. On the N-type SeZn substrate 22, there are an N-type ZnSe buffer layer 23, an N-type SeSMgZn cladding layer 24, a ZnCdSe active layer 25, a P-type SeSMgZn cladding layer 26, and a P-type contact layer 27 in order. The N-type ZnSe buffer layer 23 is mainly used to match the lattice between the N-type SeZn substrate 22 and the N-type SeSMgZn cladding layer 24. Both the N-type SeSMgZn cladding layer 24 and the P-type SeSMgZn cladding layer 26 have a broader band gap then the ZnCdSe active layer 25. Therefore, the electrons produced in the ZnCdSe active layer 25 are limited therein.
The LED of FIG. 2 further comprises an N-type electrode 21 formed on the N-type SeZn substrate 22 and a P-type electrode 28 formed on the P-type contact layer 27. When appropriate voltages are applied to the N-type electrode 21 and the P-type electrode 28, the ZnCdSe active layer 25 located on the P-N junction will emit blue light. The blue light is partially absorbed by the N-type SeZn substrate 22, from which yellow light is emitted. Part of the blue light, unabsorbed by the N-type SeZn substrate 22, is mixed with the yellow light, and thus white light is produced.
Compared with the LED of lateral-electrode type, the LED of vertical-electrode type requires a simpler manufacturing process, wherein the steps of etching for electrodes and forming phosphor on the stack structure can be omitted. However, the LED of vertical-electrode type has a lower efficiency of emitting white light than the LED of lateral-electrode type.
Therefore, the present invention provides another LED structure for the vertical-electrode type by using a P-type ZnTe layer or a ZnSe layer as a substrate. To ensure lattice match between the substrate and the blue light LED of cubic crystal, a buffer layer of single crystal is firstly formed on the substrate. Thus the efficiency of white light emission is greatly improved. Moreover, the present invention improves the quality of white light because blue light of wavelength from 450 nm to 470 nm is emitted from the blue light LED of cubic crystal on the buffer layer, and yellow light of 550 nm is emitted from the substrate after the substrate absorbs the blue light.
It is therefore an object of the invention to provide a light emitting diode (LED) structure of vertical-electrode type.
Another object of the present invention is to provide an LED structure comprising a P-type ZnTe layer or a ZnSe layer as a substrate. A BP buffer layer of single crystal is positioned on the substrate, such that the lattice match between the substrate and the blue light LED of cubic crystal can be improved.
On the substrate, a BP buffer layer, a first type GaN cladding layer, an active layer and a second type GaN cladding layer are stacked in order. A first type electrode and a second type electrode are positioned below the substrate and on the second type GaN cladding layer respectively. The first type GaN cladding layer is opposite the second type GaN cladding layer in conducting type, the first type electrode is also opposite the second type electrode in conducting type. The substrate, the BP buffer layer, the first type electrode and the first type GaN cladding layer are of the same conducting type.